Piezoelectric unit cell

ABSTRACT

The present invention is directed to a piezoelectric unit cell. The piezoelectric unit cell comprises two piezoelectric mechanically biased bender-elements placed together end-to-end and held together at their ends, the two bender-elements being mechanically biased in opposite directions such that each element is in contact only at their ends.

RELATED APPLICATION

This application is a continuation-in-part application of U.S. patentapplication Ser. No. 08/573,202, filed Dec. 15, 1995 now abandoned,entitled "PIEZOELECTRIC ELECTRO-MOTIONAL DEVICE" which is in turn acontinuation-in-part application of U.S. patent application Ser. No.08/297,233, filed Aug. 29, 1994, entitled "PIEZOELECTRIC PUMP", nowabandoned.

FIELD OF INVENTION

The present invention is directed to a piezoelectric unit cell. Morespecifically, a piezoelectric unit cell is fabricated using twomechanically biased bender-elements, each mechanically and electricallybiased in opposite directions. The ends of the bender-elements are heldtogether at their ends such that the two bender-elements deflect inopposite directions when subjected to an electrical field.

BACKGROUND OF THE INVENTION

Piezoelectric materials have been used extensively as sensors andacoustical/electric coupling devices. Materials that have been used inthese devices are made from films of polymer such as polyvinylidenefluoride (PVDF) which are drawn or stretched while subjecting thepolymer film to an electric field. The piezoelectric film will thenrespond to applied electrical fields by either lengthening or shorteningdepending upon the direction of the applied field. The deflection whichcan be obtained using piezoelectric polymer films are substantiallygreater than those obtained using piezoelectric ceramic crystals.

There are several specific techniques disclosed for making thesensor-elements using piezoelectric films; however, common to thosefolding the piezoelectric polymer film in multi-layers is the use of anepoxy resin or a glue as an adhesive between film layers. Papersdisclosing making sensors using bimorph elements and specific techniquesin making the elements are: "Application of PVF₂ Bimorph CantileverElements to Display Devices", M. Toda and S. Osaka, Proceeding of theS.I.D., Vol 19/2, Second Quarter 1978, pp 35-41; "Electro-motionalDevice Using PVF₂ Multilayer Bimorph", M. Toda and S. Osaka,Transactions of the IECE of Japan, Vol E61 No 7, July 1978, pp 507-512;"Theory of Air Flow Generation By a Resonant Type PVF₂ BimorphCantilever Vibrator", M. Toda, Piezoelectrics, 1979, Vol 22, pp 911-918;"Voltage-Induced Large Amplitude Bending Device--PVF₂ Bimorph--ItsProperties and Applications", M. Toda, Piezoelectrics, 1981, Vol 32,pp127-133; and "The Potential of Corrugated PVDF Bimorphs for Actuationand Sensing", Gale E. Nevil, Jr. and Alan F. Davis, SMEConference--Robotics Research: The Next Five Years and Beyond, Aug.14-16, 1984, Technical Paper MS84-491. When multi-layer piezoelectricpolymer film elements were made "the films were bounded together usingepoxi-resin (High Super, Cemedine Corp.)" {"Electromotional DevicesUsing PVF₂ Multilayer Bimorph", sic. p 509}.

The following patents are all patents of Toda et al. which disclosebimorph elements of piezoelectric materials. U.S. Pat. No. 4,162,511discloses a pickup cartridge for use in a velocity correction systemwhich includes a polymer bimorph element mechanically interposed betweena cartridge housing and a pickup arm carrying a groove-riding stylus.U.S. Pat. No. 4,164,756 discloses a signal pickup stylus whichcooperates with an information storing spiral groove on a video discrecord which is caused to selectively skip groove convolutions of thedisc record to produce special effects. U.S. Pat. No. 4,176 378discloses a pickup arm pivotally coupled to a housing support at one endthereof and which is coupled to the housing near its other end by meansof bimorph elements attached together at right angles. U.S. Pat. No.4,234,245 discloses a light control device which includes a bimorphelement comprising two thin polyvinylidene fluoride films and a thinlayer disposed therebetween to secure the films together. U.S. Pat. No.4,351,192 discloses a piezoelectric, acoustic vibration detectingelement which is positioned in a fluid flow to be measured so as to bemoved according to the intensity of the fluid flow away from a source ofacoustic vibration. U.S. Pat. No. 4,417,169 discloses a photoelectriccircuit arrangement for driving a piezoelectric bimorph element to bendand thereby to open or close a window blind according to the quantity oftransmitted light through the blind.

U.S. Pat. No. 4,342,936 discloses a piezoelectric flexure mode device(called a "unimorph") comprising a layer of piezoelectric activematerial bonded to a layer of piezoelectric inactive material.

U.S. Pat. No. 4,405,402 discloses a thick piezoelectric/pyroelectricelement made from polarized plastics such as polyvinylidene fluoride.

U.S. Pat. No. 4,670,074 discloses a composite co-laminated piezoelectrictransducer with at least one layer of polymeric substance capable ofacquiring piezoelectric properties when co-laminated in the presence ofan electric field.

U.S. Pat. No. 4,708,600 discloses a piezoelectric fluid pumpingapparatus which includes a pumping apparatus incorporating apiezoelectric energizer.

U.S. Pat. No. 4,939,405 discloses a pump comprised of a piezoelectricvibrator mounted in a casing.

U.S. Pat. No. 5,113,566 discloses a method of producing a multilayerpiezoelectric element.

SUMMARY OF THE INVENTION

The present invention is directed to a piezoelectric unit cell. Morespecifically, a piezoelectric unit cell of the present invention is madewith two mechanically biased bender-elements which are mechanically andelectrically biased in opposite directions such that they are in contactonly at their ends. The ends of the bender-elements are held together,preferably as a compliant hinge.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic series of views (a; b; c; and d) illustrating thefabrication of a bender-element using two strips of polyvinylidenefluoride having a thin layer of silver electrode coating on each side(the film being cut with tabs and the coating being the shaded layersapplied to top and bottom), the polarity of the top film ofpolyvinylidene fluoride being in opposite direction than that of thebottom film of polyvinylidene fluoride; specifically, FIG. 1(a) is onestrip of film having only the top of the tabs coated and one tab folded;FIG. 1(b) is a second strip of film having the top of one tab and thebottom of the other tab coated and one tab folded; FIG. 1(c) showsplacing the two films together; and FIG. 1(d) showing the two filmconnected;

FIG. 2 is a schematic series of views illustrating the folding of thetwo strips of piezoelectric multimorph film before bonding or laminatingthe strips; specifically, FIG. 2(a) shows that two films are connectedas fully shown in FIG. 1; FIG. 2(b) shows the geometry of the strips andthe polarity-machine orientation; and FIG. 2(c) illustrates the foldingof the films to form a bender-element of the present invention;

FIG. 3 is a cross-sectional and end view of a press with jaws having asize and shape to bond a bender-element with a desired radius ofcurvature;

FIG. 4 is a schematic illustrating the sine curve of an alternatingelectric field changing the polarity placed on a unit cell and thecorresponding deflection changes of the unit cell; specifically, FIG.4(a) illustrates the deflection of the unit cell at one extreme ofpolarity; FIG. 4(b) illustrates the unit cell with no deflection due topolarity; and FIG. 4(c) illustrates the deflection of the unit cell atthe other extreme of polarity;

FIG. 5 are schematic views illustrating the piezoelectric unit cell ofthe present invention; specifically one view, FIG. 5(i a), with anelectrical polarity which provides a field across the bender-elements ofthe unit cell and the unit cell is in the expanded state; the secondview, FIG. 5(b) in which the polarity of the electric field on the unitcell is reversed and the unit cell is in the contracted state;

FIG. 6 are schematic views of a stack or plurality of unit cells on abacking plate; specifically one view, FIG. 6(a), with an initialelectrical polarity which contracts the unit cells and the other view,FIG. 6(b), with an opposite electrical polarity providing a field acrossthe bender-elements which expands the unit cells;

FIG. 7 is a schematic view which illustrates a simple piezoelectricelectro-motional device with a plurality of unit cells acting as thedrive block for a single chamber pump, the pump in cross-section withoutthe outside housing;

FIG. 8 is a schematic view which illustrates a piezoelectric pump withparallel multi unit cells activating push-pull pistons of apiezoelectric pump with double parallel chambers;

FIG. 9 is a schematic diagram illustrating the electrical circuit tooperate the piezoelectric pump;

FIG. 10 is a schematic diagram of a unique circuit for powering the unitcells of the present invention;

FIG. 11 is a schematic view which illustrates a piezoelectric pump withparallel multi unit cells activating push-pull pistons of apiezoelectric pump with double parallel chambers and inlet and outletpulse dampers;

FIG. 12 is a schematic view of a piezoelectric pump with push-pullpistons in double parallel cylinders and inlet and outlet pulse dampers;

FIG. 13 is a schematic view of a peristalic pump with three multi cellsactivating the fluid flow through a flexible tubing; FIG. 13(a) and13(b) shows the cyclic activation of the three piezoelectric unit cellsto maintain positive flow; and

FIG. 14 is a schematic view of a piezoelectrically driven centrifugalpump.

BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENTS

The fabrication of the bender-element and the piezoelectric unit cellare unique and provide the basis of the piezoelectric devices of thepresent invention. Heretofore, piezoelectric elements have principallybeen used as sensors and the deflection movement of the element has beenthe major consideration. Thus, mechanical integrity was a minor part ofthe element. The fabrication of any multiple layer piezoelectric benderelement heretofore has employed an epoxy resin or some other adhesive tobind the layers. On the other hand, the piezoelectric bender-elementsand more specifically the piezoelectric unit cells of the presentinvention are used as driving blocks or force sources which may be usedin many applications such as a piezoelectric pump.

Heretofore, a plurality of piezoelectric elements, all working as asingle unit, have been used to increase the force of the deflection butwhile the addition of additional layers of elements increases the force,the plurality of layers or elements decreases the deflection. Toovercome this dilemma, the present invention uses mechanically biasedpiezoelectric bender-elements (meaning that the bender-elements arecurved in their fabrication). Two of these mechanically biasedpiezoelectric bender-elements are then fabricated into a unit cellwherein the two bender-elements are both mechanically and electricallybiased in opposite directions. This basic structure of the unit cell ascompared to a single piezoelectric element has at least four times thedeflection for a given drive voltage. In addition, by usingmulti-layered bender-elements in the unit cell of the present invention,the force can be multiplied while retaining the maximum deflectionpossible for a given drive voltage.

The bender-elements of the present invention are fabricated usingmultilayered films of a piezoelectric material such as a film ofpolyvinylidene fluoride. A piezoelectric film has the property that whenthe film is subjected to an electric field the film either lengthens orshortens depending upon the direction or polarity of the appliedelectrical field. A film of polyvinylidene fluoride is madepiezoelectric by drawing or stretching the film while subjecting thefilm to an electric field. In order to increase the deflection for agiven drive voltage from that of a single film layer of polyvinylidenefluoride, a two layer or bimorph bender-element is fabricated with thelayers arranged so that one layer lengthens while the other layercontracts. Since both layers are bonded together, the bimorph bends in afashion similar to a bimetallic thermostat element. In order to increaseforce and preserve deflection for a given drive voltage from that of abimorph, a multilayer or multimorph bender-element is fabricated. Toproduce a multimorph bender-element of the present invention, thefabrication method is done without the use of an epoxy or glue adhesive.The electrode coating may be a highly conductive metal, such as silveror a metal such as platinum, gold, copper or any combination ofconductive material. Piezoelectric polyvinylidene fluoride films are thepreferred materials used in the fabrication of the multimorphbender-elements. Such films are available from Amp Incorporated in filmthicknesses which range from 9 microns to 600 microns and are availablewith a silver coating.

The fabrication method of the present invention involves the steps ofbonding by heating while under pressure the layers of piezoelectricmaterial and then annealing to form the bender-elements of the presentinvention. The layers of piezoelectric material are placed in a curvedpress so that the bender-elements are fabricated with a mechanial biasor a natural curve.

A preferred embodiment of the present invention involves the folding ofthe electrode coated piezoelectric polymer films and is unique to thepresent invention. The cutting of the film and the presence ornon-presence of the electrode coating on certain portions of the cutfilm is shown in FIG. 1. A first strip 2 of polyvinylidene fluoride isshown in FIG. 1(a) and a second strip 4 of polyvinylidene fluoride isshown in FIG. 1(b). Each strip 2 and 4 have a thin layer of silverelectrode coating 6 (cross hatching) applied to each side of the strips2 and 4 in preparation to fabricate a multimorph bender-element of thepresent invention. As shown in FIG. 1(a), the first strip 2 has two tabs8 and 10 extending from the strip 2; however, only the top of tabs 8 and10 are coated with the silver coating 6 and neither of the bottomsurfaces of tabs 8 and 10 have any silver electrode coating 6. The tab10 when fabricating the bender-element of the present invention isfolded or bent downward as shown in the bottom figure of FIG. 1(a). Thestrip 4 on the other hand has two tabs 12 and 14 which are positionedopposite that of the tabs 8 and 10 of strip 2 as shown in FIG. 1(a). Thetop surface of tab 12 and the bottom surface of tab 14 have a thin layerof the silver electrode coating; whereas, neither the bottom surface oftab 12 or the top surface of tab 14 have any silver electrode coating asshown in the upper figure of FIG. 1(b). The tab 14 when fabricating thebender-element of the present invention is folded or bent upward asshown in the lower figure of FIG. 1(b). The two strips 2 and 4 are thenplaced one on top of the other as shown in FIG. 1(c). The tabs 8 and 12extend from the end of the two strips and the electric wires orconnections from an electrical circuit are connected to each of thesetabs 8 and 12. The tabs 10 and 14 on the other hand are folded toprovide electrical contact with the reverse side of the respective stripas shown in FIG. 1(d). Sheets of piezoelectric film are available withan electrode coating already applied to both surfaces of the film. It ispreferred that the electrode coating be removed at the edges of strips 2and 4 as well as removing the coating from the tabs as indicated whencutting the strips from already coated polyvinylidene fluoride or PVF₂strips. Removing the conductive silver electrode material from the edgesprevents high voltage arcing.

The first step in fabricating the bender-elements of the preferredembodiment of the present invention is to fold at least two strips 2 and4 of the film as illustrated in FIG. 1. The strips 2 and 4 of coatedfilm are folded as shown in FIG. 2. The first strip 2 is folded intolayers (2 shown and can extend to any number desired) to produce amulti-layered bender-element. The polarity-machine orientation of thestrip 2 of piezoelectric film is opposite that of the strip 4 ofpiezoelectric film when a voltage is applied to the films or thepolarity directions of the film are opposite. What oppositepolarity-machine orientation means is that when an applied voltage isapplied to the films, the field voltage is in a direction which is thesame as the polymer orientation and the one film will expand while thefield voltage is opposite the polymer orientation and the other filmwill contract. The arrow 16 shows the polarity-machine orientation orpolarity of the strip 2. The second strip 4 is folded into the samelength and same number of layers as strip 2 but the polarity of thefilm, as shown by the arrow 18, is in the other direction. In otherwords the polarity-machine orientation of the second strip 4 is 18° fromthe first strip 2 and therefore one film will expand while the otherwill contract. At this point, it is pointed out that polarity will bediscussed in two ways in the understanding of the present invention: (1)the applied voltage polarity which is a function of the electricalcircuit connected to the strips and (2) the inherent polarity or polymerorientation of the strips 2 and 4 which is the function of the machinedirection of the respective strip (indicated by an x or in a circle anda machine direction away from the tabs). Thus, when the voltage polarityis reversed on the tabs 8 and 12, it reverses the polarity-machineorientation of the films such that the film that expanded will contractand the film that contracted will expand.

The uniqueness of the tabs 8, 10, 12 and 14 is that they provide thecontinuity of applied polarity to the multimorph or folded structureshown in FIG. 2 through a single set of leads attached to tab 8 and tab12. Thus, from a single pair of electrical leads one of positivepolarity and the other negative, the tabs provide opposite polarities tothe electrode film surfaces of the two strips 2 and 4. For example,assuming that the polarity to tab 8 is positive and thus the top surfaceof strip 2 is positive, tab 10 provides the same polarity to the bottomsurface of strip 4 when that strip is folded back over the tab 10 asshown in FIG. 2. Assuming tab 8 is positive then tab 12 is negative andthe same negative polarity is on the top surface of strip 4 and thebottom surface of strip 2 and that polarity continues however manynumber of layers the strips are folded. The tab 14 is redundant as torequiring this tab to provide the same polarity from the bottom surfaceof strip 4 to the top surface of strip 2; however, the two tabs 10 and14 provide a greater surface area for the flow of electrons to providethe same polarity to these two surfaces. A restricted path for the flowof electrons may cause a hot spot or short. The uniqueness of the tabsand the folding is that only two leads are required.

A multi-layer bender-element may be made without all the specifics ofthe preferred embodiment. For example, the piezoelectric material neednot be solely strips of polyvinylidene fluoride film coated with silveras the electrode coating. To make the multi-layer bender-element of thepresent invention the orientation of the layers of electrode coatedpiezoelectric material need to be the same as a single folded film. Inother words, if the piezoelectric material has a polarity-machineorientation, the respective layers will have the same orientation as asingle folded film. Or stated still in another way, the respectivelayers of material can not be simply randomly stacked. As an alternativeto tabs, the discontinuous piezoelectric film or material may have smallopening extending though the layers of piezoelectric material forelectron flow.

Referring now to FIG. 3, to laminate the strips of film, the foldedstrips 2 and 4 of film are positioned into a press 20 having an upperjaw 22 and lower jaw 24, preferably each jaw made of machined pieces ofpolycarbonate. A preferred set of jaws 22 and 24 have a slight radius ofcurvature or curved portion 26 to fabricate the bender-elements with amechanical curvature or bias. The two folded strips 2 and 4, as shown inFIG. 2(c), are positioned between upper jaw 22 and lower jaw 24. Thejaws 22 and 24 of the press 20 are closed and as much pressure asrequired is applied to the two separate folded films. The pressure mayrange from 100 pounds per square inch (psi) to 10,000 psi. The press 20and the compressed films are then subjected to a heating cycle to bondthe films, such as placing the compressed films into a low temperatureoven. The temperature of the oven may range from 35° C. (95° F.) to 65°C.(149° F.). At the higher temperatures the compressed films in press 20are left in the oven for a shorter time, approximately a half hour,while at the lowest temperatures the press 20 will be kept in the ovenfor as long as 12 hours. The press 20 is then removed from the oven andwithout removing the compression on the films, is air cooled to roomtemperature. The bonded and annealed films are removed from the vice asa multi-layered bender-element 30 having a desired mechanical bias orcurved shape. After removing the bender-element 30 from the vice 20, thecontinuity of the multimorph bender-element is tested. A simple test isto apply an electrical field and if the multi-layered or multimorphbender-element expands or contracts then the bender-element has thedesired electrical continuity. As shown in FIG. 4, the natural state ofthe bonded bender-element 30 is that of FIG. 4(b), i.e. having acurvature or mechanical bias such as shown. When the polarity is in onedirection, the bender-element as shown in FIG. 4(a) is in the expandedstate and when the polarity is reversed, the bender-element as shown inFIG. 4(c) is in the contracted state. The multi-layered bender-element30 from an electrical viewpoint acts as a capacitor and resistor in theelectrical circuit.

The configuration of a piezoelectric unit cell 40 is illustrated in FIG.5. In the preferred embodiment, at least two multi-layeredbender-elements 30 are placed end-to-end, specifically bender-element 32and 34, with the ends held together with a compliant hinge 36 and themechanical bias or curvature of each bender-element is in the oppositedirection. The unit cell 40 in which the bender-elements 32 and 34 arein the contracted state is shown in FIG. 5b. It becomes clear that thefabricated bender-elements 30 must have a bias when fabricated to make aunit cell 40 so that when the polarity of the field across each of thebender-elements 30 results in the bender-elements being in theircontracted state, the two bender-elements will not come into contactwith one another. Stated differently, a unit cell 40 of the presentinvention has a greater deflection potential than if only one polaritycan be placed on bender-elements 30 of a unit cell 40. FIG. 5(a)illustrates the unit cells 40 with an opposite field polarity acrossbender-element 32 and bender-element 34. The advantage of having twobiased or curved bender-elements is that when subjected to an electricalfield the unit cell 40 has much greater deflection than a singlebender-element. When the current applied to the unit cell 40 alternatesin polarity, illustrated by the sine wave 42 shown in FIG. 4, or thepolarity of the field across the two bender-elements is reversed usingthe same voltage, the unit cell 40 will expand as shown in FIG. 5(a). Itcan be seen that when the voltage polarity on the unit cell 40 isreversed from that shown in FIG. 5(a), the unit cell 40 in FIG. 5(b)becomes almost flat, thus obtaining the greatest deflection between thetwo peaks of the sine wave 42. Without the bias or curvature at the restposition of the bender-elements 32 and 34 which make the unit cell 30, acircuit which reverses the field on the unit cell cannot be used.Therefore, without increasing the magnitude of the voltage used, butreversing the polarity, the deflection of the unit cell 40 can bedoubled. This enables the unit cell 40 of the present invention to havea much greater application of uses. This configuration of twobender-elements held together with the mechanical and electrical bias inopposite directions is the prime aspect of the unit cell of the presentinvention regardless of the construction of the bender-elements whetheruni-morph or multi-morph.

The upper bender-element 32 and the lower bender-element 34 of unit cell40 are held together with a compliant hinge 36 such as a piece of tape.The hinge 36 may be on the inside of the two bender-elements 32 and 34as shown in FIG. 5 or may be on the outside of the two bender-elements32 and 34, such as a piece of tape stuck to the upper surface of the topbender-element 32 and to the lower surface of the bottom bender-element34 or a hinge of comparable design may be used. When an electrical fieldis placed across the two bender-elements 32 and 34 of the unit cell 40,the bender-elements deflect in the opposite direction. In the same fielddue to the folding of the strips 2 and 4, the opposite polarity of thestrips 2 and 4 of piezoelectric films in the upper bender-element 32will cause one film to expand while the other film will contract, forexample, the uppermost strip of film therein may expand while the lowerstrip of film in the same bender-element 32 will contract. Likewise, theopposite polarity of the strips of film in the lower bender-element 34will cause the lowermost strip of film therein to expand while the upperstrip of film in the same bender-element 34 will contract. It is notedthat by reversing the polarities of the strips of film 2 and 4 in thesame bender-element 30 and the manner in which the films are folded thata single polarity field increases the deflection within a singlebender-element 30, rather than requiring two fields in the oppositedirection across films to obtain the greatest deflection. Further, onlya single field is required for the unit cell 40, since the twobender-elements 32 and 34 are electrically in parallel, to obtain thedesired maximum deflection of the unit cell 40. Preferably, apiezoelectric unit cell 40 is symmetrical having the same number offolds in each of the bender-elements 30 of the top bender-element 32 andthe bottom bender-element 34. However, an asymmetrical unit cell 40 mayalso be fabricated. The unit cell 40 has an application for any linearmotion use.

Referring now to FIG. 6, the linear electro-motional application of aunit cell 40 is illustrated. However,instead of using a single unit cell40, a plurality of unit cells 40 may be stacked one on the other toobtain a greater displacement per unit force when the plurality of cells40 are subjected to an electrical field and deflection of each unit cell40 occurs. The unit cells 40 are shown stacked on a backing plate 44.This structure of a plurality of unit cells 40 and a backing plate 44-is basic to many alternatives for the remaining structure to which theunit cells 40 are put to use. For example, when the force of thedeflection of the unit cells 40 is desired in a specific direction, thebacking plate 44 may represent a fixed structure from which thedeflection occurs. On the other hand, the stack of unit cells 40 mayhave a movable member extending across the top of the stack and thebacking plate 44 represents such a member, for example a membrane or apiston actuator which will receive the force of the deflection and movewith the upper surface of the top unit cell as the field is applied andremoved or the polarity of the field is reversed. Still further, if thestack of unit cells 40 have a fixed upper structure, the deflection willcause a force on the backing plate 44 to move downward and representsthe movable structure or the structure against which the force isapplied. It is apparent that there are many variations which are readilypossible to benefit from the deflection of the stack of unit cells 40and therefore the force of the plurality of unit cells 40.

One specific electro-motional embodiment is a piezoelectric pump asshown in FIG. 7. The pump 50 in its simplest form has a housing (notshown) with a drive block chamber 52 containing side-by-side unit cells40 and preferably a plurality or stack of unit cells 40. At the top ofchamber 52 is a diaphragm 54. The unit cells 40 may be in direct contactwith the diaphragm 54 or as shown are in contact with a piston 56. Anaccumulator chamber 58 is at the top portion of the housing of pump 50.A fluid inlet 60 has an inlet check valve 61 for fluid entering theaccumulator chamber 58. At the outlet of accumulator chamber 58 is afluid outlet 62 having an outlet check valve 63. As shown, the unitcells 40 are in their expanded state causing an upward force to beapplied to the piston 56 and diaphragm 54 forcing the fluid out of theaccumulator chamber 58. When the polarity of the field across the unitcells 40 is reversed, the unit cells 40 contract from the position shownand remove the force on the diaphragm 54 permitting fluid to flow intothe chamber 58.

The piezoelectric pumps of the present invention can have a variety ofconfigurations. For example, a multichambered pump with chambers inseries or multichambered pump with chambers in parallel or combinationsthereof. A multichambered pump 70 is shown in FIG. 8 which operates inthe same manner as the single chamber pump except that while fluidenters one chamber the fluid in the other chamber in being forced out.The pump 70 is illustrated as having two chambers and a "push-pull"arrangement of the piezoelectric unit cells which operate on both sidesof the drive piston 72. The lower unit cells 40L are driven by the sameelectronic signal as the top unit cells 40T; however, the polarity ofthe lower unit cells is opposite that of the upper unit cells. Theadvantage is that the entire capacitance of the system, including bothupper and lower unit cells is incorporated into the electronic drivecircuit. This results in a highly accurate timing system. Anotheradvantage is that as the field polarity is reversed, the contractingunit cells are putting work into the system as well as the expandingunit cells. Depending on the use of the pump, a variety of electriccircuits may be used to provide the field to the unit cells 40 (T andL). A direct drive circuit would provide an on-off field to the unitcells. An alternative to using a direct drive circuit is to employ aparallel resonate drive circuit. The parallel resonate circuit, whendriven by a sine wave, allows the phase angle between the drive voltageand current to approach 90 degrees. Power is defined as the product ofthe voltage and the current. When the sine wave phase angle between thevoltage and the current approaches 90 degrees, the power required tomaintain the oscillation is at a minimum. Application of a parallelresonate circuit reduces the power required to operate the system, andtherefore increases system efficiency. This is accomplished using acircuit configuration that takes advantage of the capacitive nature ofthe unit cells 40 (T and L). The capacitance of the unit cells is usedin conjunction with an inductance to produce a tuned LC parallelresonate circuit where the L refers to a measure of inductance and Crefers to a measure of capacitance. Preferably the inductance issupplied to the circuit in the form of a step-up transformer. Thestep-up transformer being required to boost the supply voltage to arange appropriate for driving piezoelectric unit cells. Typicallyresonant circuits are avoided when building control circuits forpiezoelectric films because of the narrow frequency response of theresulting circuit and because most applications of piezoelectric filmsare as sensors, which generally need to operate over a wide range offrequencies. A resonant circuit is not a problem for mechanical powerapplications, such as a pump, because the operating frequency of thedrive circuit is fixed to optimize the desired mechanical output of thebender-elements. Once the drive frequency is established, the LC circuitcan be designed precisely to the mechanical frequency required.

A piezoelectric pump 80 with an electrical circuit diagram isillustrated in FIG. 9. The electrical diagram shown utilizes theinherent inductance of a transformer 81 as part of the tuned resonanttank 82. This electrical diagram allows the resonant frequency of thetuned resonant tank 82 to be adjusted using a low voltage capacitor inthe drive module 83 across the primary of the transformer 81 rather thanhaving to add inductors or high voltage capacitors across thepiezoelectric pump 80.

The present invention is more fully set forth and illustrated by thefollowing examples:

EXAMPLE I

A 1.1 mil thick sheet of polyvinylidene fluoride (Amp Incorporated)coated with silver ink is labeled and cut into two strips. The edges ofthe strips are masked off with 3M soft stick tape and the border ofsilver ink is removed with methyl-ethyl ketone (MEK). The two strips arecarefully folded (eight folds) as shown in FIG. 2, with thepolarity-machine orientations of the strips in opposite directions. Thetwo strips are placed into a vice with polycarbonate jaws. The vice isclosed applying as much pressure as possible. The vice is placed in anoven and heated at 122° F. (50° C.) for ten hours to bond the silver inklayers. Without removing the pressure, the vice is removed from the ovenand allowed to air cool to room temperature.

The bonded and annealed bender-element is removed from the vice. Thebender-element is tested for continuity of the multimorph by applying afield on the bender-element and observing the deflection.

This example illustrates the method of fabricating the bender-elementsof the present invention.

EXAMPLE II

Following the same procedure as set forth in Example I, the folded filmsare inserted into a vice where the jaws have been machined such thatthey have a curvature as illustrated in FIG. 3.

This example illustrates the method of fabricating the biasedbender-elements of the present invention.

EXAMPLE III

A pair of bender-elements fabricated by the method of Example II areplaced in juxtaposition to one another such that an applied field willcause the deflection to be in opposite directions. The ends of eachbiased bender-element is fixed to the corresponding ends with Scotchtape. An applied field causes the deflection of the pair ofbender-elements as shown in FIG. 5.

This example illustrates the piezoelectric unit cell of the presentinvention.

The piezoelectric unit cells of the present invention have a widepotential of uses. The configuration of a pump 80 and the circuitdiagram as illustrated in FIG. 9 is suited as a liquid coolingventilation garment (LCVG) pump. In addition to the active thermalcooling application of the LCVG pump, piezoelectric pumps can act aselectromechanical actuators. As an actuator, the piezoelectric pump mayprovide solutions to control problems in robotics, bioengineering,advanced remote control and telepresence technologies.

The piezoelectric electromechanical device of the present inventionbesides being used in a pump may be used as an actuator, such as anylinear short stroke actuator, which may fill the demand for outputdevices that are more energy efficient, rugged, economical and easier tocontrol than conventional actuators.

The present invention also includes a unique circuit for thepiezoelectric (piezo) film drive circuit shown in FIG. 10. The key tothe circuit system lies in its ability to transfer energy from thecharged piezo film, transfer the energy to an inductor and recharge thepiezo film with the opposing polarity all at frequencies which providethe desired maximum energy to be applied to the film or morespecifically the unit cell(s). The frequency is controlled by the use ofa triac and triac driver in the circuit which will be explained inreference to FIG. 10. As mentioned hereinbefore, the piezo film (unitcell or cells) acts as a resistor and capacitor, shown as R1 and C1 inthe circuit. A power source, illustrated as 450 volts DC, is used toinitially charge the film. This is accomplished by the control circuitturning on Q1, or closing the circuit as illustrated, and allowing thepiezo film to charge to 450 volts (v). The charge current, and hence thecharge time, is controlled by cycling Q1 on and off (e.g. 2 kHz). Theduty cycle is set so as not to exceed the maximum allowable currentavailable from the power source. The inductance of L2 is used to reducethe initial spike in current during each recharge cycle as will beexplained in more detail hereinafter.

Before explaining the further operation of the circuit, the circuitsother components are a triac X1 which acts as a gate to a storageinductor, L1 and R3; a triac driver U1 operated by an opto-isolator witha pulse signal V1; and a replenish control.

With the piezo film charged, the oscillation is initiated. The timingpulses required to set the frequency are TTL level signals with a pulsewidth of 10 μs or less delivered at twice the desired drive frequency.The narrow pulse width is required so that the triac is allowed to turnoff when the current in the inductor reaches zero. The control signal isrepresented as V1 in the schematic and supplies the drive current to theopto-isolator which, in turn, provides the switch on signal for thetriac gate.

The first pulse occurs after Q1 is opened. When the first pulse occurs,the triac X1 is turned on and current begins to flow from the piezo filmthrough the triac X1 and the main inductor L1. As the current magnitudeincreases above the minimum hold current for the triac, the triac islatched on and will continue to conduct until the current drops belowthe minimum hold current (near zero), at which point triac X1 willswitch off. The voltage present on the piezo film during this time fromthe triac being turned on to off has gone from a positive peak (+450 v)to a negative peak (near -450 v). The polarity reversal is provided bythe inductor. The actual voltage of the negative peak is determined bythe amount of energy lost in the inductor and piezo film during thecycle. With the triac X1 off, the piezo film will remain in itsnegatively charged state with only parasitic dielectric losses slowlyreducing the voltage present on the piezo film.

The piezo film remains in this negatively charged state until a next(second) pulse from V1. The second pulse again turns on triac X1;however, the current flow and voltages will be reversed and the processis reversed. At the end of this cycle the piezo film is left positivelycharged (somewhat below +450 v) from its previous negatively chargedstate. Again, the actual positive peak voltage is determined by theamount of energy lost in the inductor and piezo film in the two cyclesof the triac X1 being on and off.

If this process were allowed to continue, the voltages would continue todecay and the system would come to a halt after a number of cycles. Inorder to provide a continuous drive signal, the energy lost during eachtwo pulse cycles must be replenished. This is accomplished by using thecontrol circuit to turn on Q1 and using the power source to charge thepiezo film to the positive peak (+450 v). The control circuit senses thelarge positive voltage which occurs at triac X1 to turn on Q1. The turnon of Q1 replenishs or energizes the circuit to maximum voltage and theturn off of Q1 is accomplished before the next (third) pulse from V1.The third pulse initiates the next cycle which is then repeated andrepeated.

At a 60 Hz pulse drive rate, the period of the "hold time" issufficiently long to allow the piezo film to be charged back to 450 vusing relatively low charging currents. By charging only on the positiveportion of the cycle, a slight DC offset will be induced; however, ingeneral it will be a small percentage of the drive voltage and shouldnot effect the operation of the piezo film.

This unique circuit has the capability of powering any capacitive devicewhich requires the voltage of the device to alternate polarity (positiveto negative) while recovering the charging energy and at controlledfrequencies. This use of a triac is different than in applications whereit is normally used.

While the configuration of the pumps illustrated herein above arecharacterized as diaphram pumps or double action piston pumps, theversatility of the piezoelectric unit cells of the present invention areillustrated in piezoelectric peristaltic pumps and centrifugal pumps.Further, the specific pump structure may be modified for specificapplications. For example, referring to FIG. 11, double-acting diaphrampump 70 is shown with an inlet pulse dampener 90 and an outlet dampener91. These dampeners are essential to allow the pump 70 to operatebetween a relatively uniform pressure difference if it is to operatewell at resonance. Flow rates and pressures of piezoelelectric pumps arelimited only by the size which can be economically made. Small pumpswhich operate in the 0-50 psi and 0-5 gpm (gallons per minute) range arenormal.

Referring to FIG. 12, the configuration of the fluid chambers arecylinders 93 and 94 respectively. This pump is essentially a positivedisplacement pump.

Referring to FIG. 13, a peristaltic pump 95 has three piezoelectic unitcells 96, 96(a) and 96(b). A flexible tubing or bladder 97 carries thefluid being pumped. The tubing 97 is within a larger tubing or chamberhaving surfaces 98 and 99. A unit cell 96, not electrically activated,and the tubing 97 fit between the surfaces 98 and 99 without compressingthe flexible tube 97. In the operation of the pump, the unit cells 96,96(a) and 96(b) are operated sequentially in an alternating mode ofnegative (contracted position) and positive (expanded position) At thebeginning of each cycle unit cell 96 is in the negative mode whereasunit cells 96(a) and 96(b) are in the positive mode such as shown inFIG. 13. Thereafter, unit cell 96 is switched positive and unit cell96(a) is switched negative as shown in FIG. 13(a). Still further, unitcell 96(a) is switched positive and unit cell 96(b) is switched negativeas shown in FIG. 13(b). This cycle is repeated to operate the pump 95.It is noted in this pump configuration that the unit cells 96, 96(a) and96(b) each provide direct force and not indirect force as through apiston.

Referring to FIG. 14, the piezoelectric cell may be used to power aforce actuator where the force actuator is illustrated by a rack andpinion. Centrifugal pump 100 has a centrifugal pump head 102 with anoutlet 103. The inlet is opposite the drive mechanism of pump 100. Thepump 100 has a drive shaft 104 which is attached to the impeller in thepump head 102. Between the outside surface of pump head 102 and thepinion 105 on the drive shaft 104 is a unidirectional clutch (notshown). A piezoelectric unit cell 106 is affixed to a surface 108. Ontop of the unit cell 106 is a rack 110. In the operation of the pump100, the expansion of the unit cell 106 moves the rack 110 upwardsrotating the pinion 106 counter-clockwise and rotates the drive shaft104. When the unit cell 106 moves to its normal state, rack 110 movesdownward and pinion 105 rotates clockwise but drive shaft 104 does notrotate since the clutch is not engaged. These examples of differenttypes of pumps illustrate the versatality of the kinds of pumps whichare available and the various operations of the unit cells of thepresent invention. The pumps may have applications as a heart pump,metering pump for medications or numerous other applications.

I claim:
 1. A piezoelectric unit cell which comprises:a first piezoelectric bender-element, said bender-element being multi-layer and mechanically biased; a second piezoelectric bender-element, said bender-element being multi-layer and mechanically biased, said first and second bender-elements being placed end-to-end whereby said first piezoelectric bender-element is mechanically biased in one direction and said second piezoelectric bender-element is mechanically biased in an opposite direction of said first bender-element; and means for preventing separation of said ends of said two bender-elements.
 2. A piezoelectric electro-motional device which comprises:a piezoelectric unit cell comprisinga) a first biased multi-layer piezoelectric bender-element; b) a second biased multi-layer piezoelectric bender-element, each said bender-element placed end-to-end and being mechanically biased in opposite directions; and c) means for preventing separation of said ends of said two bender-elements; and means for subjecting said bender-elements to an electric field.
 3. A piezoelectric unit cell which comprises:a first piezoelectric bender-element, said bender-element being multi-layer and mechanically biased in one direction; a second piezoelectric bender-element, said bender-element being multi-layer and mechanically biased in an opposite direction of said first bender-element; each multi-layer bender-element is comprised of a single piece of material which is folded to form the layers and said first and second bender-elements being placed end-to-end whereby said first piezoelectric bender-element is biased in one direction and said second piezoelectric bender-element is mechanically biased in an opposite direction of said first bender-element; and means for preventing separation of said ends of said two bender-elements.
 4. A piezoelectric unit cell according to claim 1 wherein said means is a compliant hinge.
 5. A piezoelectric unit cell according to claim 4 wherein said compliant hinge is a piece of tape. 